The present invention generally relates to an atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which is selectively operable in a test mode for setting a threshold level for the cardioverting electrical energy. More specifically, the atrial defibrillator of the present invention, when the test mode is enabled by a physician, induces atrial fibrillation when the atria are not in fibrillation and thereafter repeatedly applies cardioverting electrical energy to the atria with incrementally increased quantities of electrical energy until the atria are successfully cardioverted. Thereafter, a value indicative of the last applied quantity of energy is stored in memory for reference during subsequent normal operation.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchrony with a detected electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has precluded these defibrillators from becoming a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator with an external magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved safety are fully described in copending U.S. applications, Ser. No. 07/685,130, filed Apr. 12, 1991, in the names of John M. Adams and Clifton A. Alferness for IMPROVED ATRIAL DEFIBRILLATOR AND METHOD and Ser, No. 07/856,514, filed Mar. 24, 1992, in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen for IMPROVED ATRIAL DEFIBRILLATOR, LEAD SYSTEMS, AND METHOD, which applications are assigned to the assignee of the present invention and incorporated herein by reference. The atrial defibrillators disclosed in the referenced applications include an atrial fibrillation detector which automatically detects when the atria are in fibrillation and in need of cardioversion and a cardioverter stage which, responsive to the atrial fibrillation detector detecting atria fibrillation, automatically delivers cardioverting electrical energy to the atria.
Once an atrial defibrillator is implanted, it is desirable at periodic intervals to set the quantity of cardioverting electrical energy delivered to the atria at an energy level which provides a high probability of successful cardioversion but which is not excessively large. This is due to the fact that the electrical energy to cardiovert the atria may be painful to the patient or at least cause discomfort. Hence, it is most desirable to set the energy level to just above a threshold level which will provide the high probability of successful cardioversion. Since the threshold level will be different for each patient and changes over time, it is necessary for the physician to induce atrial fibrillation in a patient and then perform a series of cardioverting electrical energy applications to the atria at incrementally increasing energy levels until the threshold level is determined by the successful cardioversion of the atria. The series of applications is preferably started at a low energy believed to be insufficient to cardiovert the atria of the patient.
One method of inducing atrial fibrillation in a patient is to apply rapid pacing pulses to the atria or apply a series of premature pacing pulses to the atria of the patient. This approach however has severe ramifications.
Firstly, if the rapid pacing pulses are provided by the atrial defibrillator through the atrial defibrillator implanted lead system, the energy required to induce atrial fibrillation may cause excessive drain on the depletable power source, such as a battery, which supplies power to the implanted defibrillator. This excessive drain can prematurely deplete the atrial defibrillator battery and require more frequent replacement of the atrial defibrillator than desired. If the pacing pulses are not applied by the implanted defibrillator, but by an external pacing source, an additional catheter lead is required necessitating an invasive procedure to at least temporarily implant the additional catheter. This not only causes inconvenience and discomfort to the patient but additionally imposes mobility restraints of more than four hours on the patient and the concomitant mortality associated with such a procedure. Furthermore, with such rapid atrial pacing, there is always the probability of inducing undesirable other arrhythmias of the heart.
The atrial defibrillator of the present invention overcomes the disadvantages expressed above with respect to the above described method of inducing atrial fibrillation for the purpose of permitting the determination of a patient's atrial defibrillation energy threshold level. The atrial defibrillator of the present invention is selectively operable, by a physician, into a test mode wherein, in the absence of natural atrial fibrillation, electrical energy is applied to the atria having a quantity less than the quantity required for atrial defibrillation or cardioversion and at a time during the cardiac cycle of the heart when the heart is most vulnerable to the inducement of atrial fibrillation. More particularly, the electrical energy is applied in timed relation to a sensed electrical activation (R wave) of the heart and preferably in synchrony with a sensed R wave which, in most patients, occurs in time closely to the relative refractory of the atria. The atrial fibrillation inducing electrical energy is thus applied at a time during the cardiac cycle of the heart when the heart is most vulnerable to induced atrial fibrillation. In general, only one such application of electrical energy will be required for inducing atrial fibrillation. However, if more than one such application is required, the atrial defibrillator provides additional atrial fibrillation inducing electrical energy applications to the atria until atrial fibrillation is induced. Even if more than one such application is required, since the energy is applied when the heart is most vulnerable to induced atrial fibrillation, only a limited number of such applications will be required. This reduces the amount of total electrical energy required to induce atrial fibrillation as compared to the total energy required with rapid atrial pacing to accomplish this purpose. Hence, the atrial fibrillation is induced by the atrial defibrillator and its implanted lead system to thus negate the need for an invasive procedure for the temporary implantation of an additional catheter. Also, since the energy is applied in synchrony with an R wave, the inducement of ventricular fibrillation or other undesirable arrhythmias is avoided.
As will be seen hereinafter, and in accordance with the present invention, when atrial fibrillation is successfully induced, the atrial defibrillator, while still in the test mode, provides a series of cardioverting electrical energy applications to the atria. The cardioverting applications begin at an energy level which is lower than that expected for successful cardioversion and the energy level is incrementally increased for each application. When cardioversion of the atria is successful, a value corresponding to the last quantity of cardioverting electrical energy applied to the atria is stored in a memory for later reference in treating natural atrial fibrillation and the atrial defibrillator exits the test mode and reenters its normal mode of operation for cardioverting the atria when the atria are in need of cardioversion.